The use of printbars comprised of discrete, light-emitting sources in image recording systems is well known. One commonly used printbar is the LED printbar. LED printbars used in imaging systems are typically comprised of a large number of closely spaced light emitting diodes (LEDs), an array of selfoc (self-focusing) lens, an LED current driver for each LED, and control electronics (which may include a shift register). The control electronics receives image signals from an external device and routes signals to the LED current drivers so as to cause the LEDs selected by the image signals to radiate light. The selfoc lenses focus radiated light onto a photoreceptor. By providing relative motion between the LED printbar and a photoreceptor, and by applying image signals at the proper times, a desired latent electrostatic image can be produced on the photoreceptor. Each area of the photoreceptor which can be illuminated by an LED is referred to as a pixel; the sum of all pixels comprise the latent image.
The LEDs of most LED printbars are arranged in a linear array of one or more rows. By making the length of a row as long as the image that is to be formed an LED printbar can produce a desired image line by line. Since it is difficult to produce a row of closely spaced LEDs with the required length (say 8 to 14 inches) LED chips of smaller lengths are usually butted together and interconnected to act as a single row. If more than one row is used, the various rows are usually offset in a staggard fashion.
While imaging systems which use other modulation schemes are possible, the most common way of modulating the LEDs is to turn them either fully ON or fully OFF in accordance with image signals applied to the printbar. The image signals may be from any of a number of sources, such as a raster input scanner (RIS), a computer, a facsimile machine, or a word processor. U.S. Pat. Nos. 4,689,694, issued on 25 Aug. 1987 to Yoshida; 4,706,130, issued on 10 Nov. 1987 to Yamakawa; and 5,138,337, issued on 11 Aug. 1992 to Ng are representative of prior art printhead control circuitry. Prior art exposure control systems are disclosed in U.S. Pat. No. 4,525,729, issued on 25 Jun. 1985 to Agulnek et al., and in U.S. Pat. No. 5,025,322, issued on 18 Jun. 1991 to Ng. Those prior art references are hereby incorporated by reference.
To create high quality images with an LED printbar, each of the LEDs should have the same light output when ON. To meet current image quality goals the LED light output uniformity must be within 1 or 2%. One known way of achieving such uniformity this to create and store a correction matrix of light output values, one value for each LED, during a calibration phase. The stored correction values can then be downloaded to correction circuitry each time the printer is turned on. The correction circuitry can then compensate for light output differences by controlling an electrical signal, usually the drive current, to the individual LEDs based upon the stored correction values.
While the above scheme of achieving light output uniformity is generally successful, the individual LEDs of an LED printbar may have different aging characteristics which can eventually result in unacceptable pixel-to-pixel exposure uniformity. One solution to this aging problem is to periodically scan the LED printbar with a photosensor as each LED is individually turned on. The light output from each LED is then measured and, if necessary, the stored correction value for the particular LED is updated to reflect changes in light output. While this system compensates for aging, it is rather expensive and uses valuable space near the photoreceptor. Other solutions to the aging problem are described in U.S. Pat. Nos. 5,016,027, issued on 14 May 1991 to Uebbing and 4,982,203, issued on 1 Jan. 1991 to Uebbing et al., and in co-pending U.S. patent application Ser. No. 08/514,033 entitled, "AUTOMATIC LED PRINTBAR UNIFORMITY ADJUSTMENT" which was filed on 11 Aug. 1995. However, those methods may not be optimal. Therefore, a new method of maintaining light output uniformity in an LED printbar, and thus pixel-to-pixel exposure uniformity over time, would be useful.